A process for preparing manganese nitrate solution from manganese ore containing manganese dioxide is disclosed in U.S. Pat. No. 3,780,159 by Jay Y. Welsh, the disclosure of which is incorporated herein by reference. This prior process is a cyclic process in which manganese ore containing manganese dioxide was contacted and leached with an acid solution prepared by dissolving nitrogen dioxide gas in an aqueous medium. An impure aqueous nitrate solution was recovered, and purified by treatment with an alkaline agent and then heated to decompose the manganese nitrate and obtain substantially pure manganese dioxide, nitrogen dioxide and water vapor. The nitrogen dioxide and water vapor formed by this decomposition were recycled in the prior process.
U.S. Pat. No. 1,287,041 to Kaplan is directed to the leaching of manganese dioxide with nitrogen dioxide to obtain manganese nitrate which is subsequently decomposed. The exposure of powdered manganese ore to nitrogen dioxide in the presence of water is described. Kaplan suggests the use of two or more agitated vessels through which nitrogen dioxide is passed along with some air. The purpose of the air is to oxidize any nitric oxide to nitrogen dioxide. Although Kaplan does not give sufficient detail to indicate process controls, including pH, he apparently considered nitric oxide generation to be a problem to be avoided.
Technical Paper 674 by A. L. Fox (U.S. Department of the Interior, Bureau of Mines 1945) entitled "Semi-Pilot-Plant Investigations of Nitrogen Dioxide Process for Benefication of Manganese Ores" describes a method of leaching low grade manganese ore with nitrogen dioxide to obtain a manganese nitrate solution. Three agitated tanks containing ground manganese ore are described through which nitrogen dioxide is passed in countercurrent fashion. There is no disclosure in the Fox et al. report or in U.S. Pat. No. 2,374,674 by Fox et al. of pH control during leaching of the ore. Fox et al. recycle the manganese nitrate decomposition products, nitrogen dioxide and water vapor, through a high temperature heat exchanger to effect decomposition of the manganese nitrate solution by indirect heating. It is to be noted, however, that the detailed discussion in the above-mentioned Bureau of Mines Report discloses scaling within the decomposition vessel and on the rakes employed to remove the product. This type of operation therefore appears to be commercially impractical.